--> Abstract: Three things we thought we understood about shale gas, but were afraid to ask…; #90063 (2007)

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Three things we thought we understood about shale gas, but were afraid to ask…

 

Cluff, Robert M.1, Keith W. Shanley1, Michael A. Miller2 (1) The Discovery Group, Inc, Denver, CO (2) BP America Production Co, Houston,

 

Shale-gas plays are largely evaluated and interpreted on the basis of performance data from a limited number of field areas supplemented with even more limited core data. The Barnett Shale at Newark East Field in the Ft. Worth Basin is the standard by which most shale plays are compared. We suggest this leads to a highly skewed view of these play-types and that three fundamental areas of uncertainty exist requiring additional research. (1) Significant shale-gas production is thought to occur at thermal maturities coincident with the gas window. This reflects the drop in productivity in the Barnett Shale associated with an Ro of 1.2%. Explanations offered are (1) insufficient gas generated at lower maturity levels, and (2) oil in the pore structure blocking the movement of gas. Laboratory data and modeling suggest gas is co-generated with oil and low-maturity shale plays show shales can be gas-saturated at the base of the oil-window. The idea that oil blocks gas is based on relative permeability analogies to conventional reservoirs yet realisitic relative permeabilities have not been measured in shales. (2) We think we understand the partitioning between free and adsorbed gas by subtracting helium-derived porosity from a total sorption isotherm. Helium is much smaller than methane so helium porosity greatly exceeds the space available to methane. The adsorbed-fraction is underestimated and porosity is overestimated. Because pore-sizes are much greater in conventional reservoirs this problem was not previously encountered. (3) Shales are viewed by many as extremely tight sandstones where mass transport through micropores and pore-throats move gas from matrix to fractures to well-bore. Matrix permeabilities of shales range from 500x10-6 mD to 1x10-9 mD. Effective permeabilities are almost certainly much lower.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California